Distance measuring apparatus of the foregoing kind is described, for example, in published German patent application P 31 03 567.1. There, the branching point is fixed and the light pulse emitted by the transmitter is split into two parts, one of which is emitted as a measuring light pulse to the target object and the other is fed as a reference light pulse directly to the detector equipment which is started by the reference pulse and "stopped" on reception of the reflected pulse, the time interval between starting and stopping being a dependable measure of distant targets. The problem of adapting the intensity of the emitted measuring light pulse to the reflectivity and distance of the target object and the intensity of the reference light pulse to that of the reflected measuring light pulse is solved in the noted publication by use of damping components which are arranged in the light path between the optical receiving system and detector equipment or in the reference light path and in principle can be of any desired kind. The damping components are required because the intensity of the received measuring light pulse can fluctuate within wide limits for reasons stated, and because of the effect of the light permeability of the medium lying between the measuring instrument and the target object. Because the receiver of the detector equipment has a small dynamic range in comparison with the range of the amplitude of the reflected light pulse, it is necessary for the attainment of high measurement accuracy to reduce the intensity (high for large distance measurement) of measurement, of the light pulses emitted by the transmitter for very small distances and/or for distance measurements of highly reflecting target objects. Since moreover the response behaviour of the receiver can change in dependence on the intensity of the light pulse impinging on the photo-detector therein, it is necessary for the attainment of as uniform as possible a processing of measurement light pulse and reference light pulse that the latter possesses approximately the same amplitude as the light pulse reflected by the target object. In the case of very far removed and/or badly reflecting target objects, it is therefore required correspondingly to reduce the amplitude of the reference light pulse.
Damping components are known and may comprise iris diaphragms or insertable filters. These components however are either comparatively large and slow to react and therefore unsuitable for high measurement frequencies or expensive and not readily usuable.
An objective of the invention is to provide a distance measuring instrument of the foregoing kind which is simple, reliable, operates at high speed and at low cost and which provides light pulse intensity damping means effective to damp at least one of the following (a) the light pulse transmitted to the target, (b) a reference light pulse in a reference light path and/or (c) the light pulse reflected from the target.